Just in case someone finds this in the future - squarespace support responded very quickly indicating this is a known issue and provided a work-around (until a fix is in place).
Currently we are experiencing an issue with not being able to choose a destination for the continue shopping link if the destination page is under a folder. We’ve noted your report with our Engineering and Design teams for review. They’ll use this for creating fixes and improvements for future releases.
For now, you may need to move the page you wish to link to outside of the folder it is in. This should allow you to link to it from the "Continue Shopping" link.
I tried the suggested workaround and it works!